Metal powders produced by the reduction of the oxides with gaseous magnesium

ABSTRACT

Metal powder Ta and/or Nb, with or without one or metals from the group Ta, Nb, Ti, Mo, W, V, Zr and Hf, is made in a fine powder form by reduction of metal oxide by contact with a gaseous reducing agent, preferably an alkaline earth metal, to near complete reduction, leaching, further deoxidation and agglomeration, the powder so produced being sinterable to capacitor anode form and processable to other usages.

This application is the national U.S. phase patent application ofPCT/US99/09772, filed May 5, 1999, which claims priority to U.S. Ser.No. 09/073,448 filed May 6, 1998, now U.S. Pat. No. 6,171,363-B1,granted Jan. 9, 2001, and DE 198 31 280.6, filed on Jul. 13, 1998.

FIELD AND BACKGROUND OF THE INVENTION

This invention relates to the production of tantalum, niobium and othermetal powders and their alloys by the reduction of the correspondingmetal oxide with gaseous active metals such as Mg, Ca and otherelemental and compound reducing materials, in gaseous form.

Tantalum and niobium are members of a group of metals that are difficultto isolate in the free state because of the stability of theircompounds, especially some of their oxides. A review of the methodsdeveloped to produce tantalum will serve to illustrate the history of atypical manufacturing process for these metals. Tantalum metal powderwas first produced on a commercial scale in Germany at the beginning ofthe 20^(th) Century by the reduction of the double salt, potassiumheptafluorotantalate (K₂TaF₇) with sodium. Small pieces of sodium weremixed with the tantalum containing salt and sealed into a steel tube.The tube was heated at the top with a ring burner and, after ignition,the reduction proceeded quickly down the tube. The reaction mixture wasallowed to cool and the solid mass, consisting of tantalum metal powder,unreacted K₂TaF₇ and sodium, and other products of the reduction wasremoved by hand using a chisel. The mixture was crushed and then leachedwith dilute acid to separate the tantalum from the components. Theprocess was difficult to control, dangerous, and produced a coarse,contaminated powder, but nevertheless pointed the way to what became theprincipal means of production of high purity tantalum in later years.

Commercial production of tantalum metal in the United States began inthe 1930's. A molten mixture of K₂TaF₇ containing tantalum oxide (Ta₂O₅)was electrolyzed at 700° C. in a steel retort. When the reduction wascompleted, the system was cooled and the solid mass removed from theelectrolysis cell, and then crushed and leached to separate the coarsetantalum powder from the other reaction products. The dendritic powderwas not suitable for use directly in capacitor applications.

The modern method for manufacturing tantalum was developed in the late1950's by Hellier and Martin (Hellier, E. G. and Martin, G. L., U.S.Pat. No. 2,950,185, 1960). Following Hellier and Martin, and hundreds ofsubsequently described implementations or variations, a molten mixtureof K₂TaF₇ and a diluent salt, typically NaCl, is reduced with moltensodium in a stirred reactor. Using this system, control of the importantreaction variables, such as reduction temperature, reaction rate, andreaction composition, was feasible. Over the years, the process wasrefined and perfected to the point where high quality powders withsurface area exceeding 20,000 cm²/gm are produced and materials withsurface area in the 5000-8000 cm²/gm range being typical. Themanufacturing process still requires the removal of the solid reactionproducts from the retort, separation of the tantalum powder from thesalts by leaching, and treatments like agglomeration to improve thephysical properties. Most capacitor grade tantalum powders are alsodeoxidized with magnesium to minimize the oxygen content (Albrecht, W.W., Hoppe, H., Papp, V. and Wolf, R., U.S. Pat. No. 4,537,641, 1985).Artifacts of preagglomeration of primary particles to secondary particleform and doping with materials to enhance capacitance (e.g. P, N, Si,and C) are also known today.

While the reduction of K₂TaF₇ with sodium has allowed the industry tomake high performance, high quality tantalum powders thus, according toUllmann's Encyclopedia of Industrial Chemistry, 5^(th) Edition, Volume A26, p. 80, 1993, the consumption of tantalum for capacitors had alreadyreached a level of more than 50% of the world production of tantalum ofabout 1000 tons per annum, whereas there had essentially been no use ofniobium for capacitors, even through the raw material base for niobiumis considerably broader than that for tantalum and most of thepublications on powder preparation and capacitor manufacturing methodsmention niobium as well as tantalum.

Some of the difficulties of applying that process to niobium are asfollows:

While the manufacturing process of the type shown in Hellier and Martin(U.S. Pat. No. 2,950,185) for the reduction of potassiumheptaflorotantalate by means of sodium in a salt melt is available inprinciple for the production of high purity niobium powders viapotassium heptafluoroniobate, it doesn't work well in practice. This isdue, in part, to the difficulty of precipitating the correspondingheptafluoroniobate salts and is due, in part, to the aggressivelyreactive and corrosive nature of such salts, such that niobium producedby that process is very impure. Further, niobium oxide is usuallyunstable. See, e.g., N. F. Jackson et al, Electrocomponent Science &Technology, Vol. 1, pp. 27-37 (1974).

Accordingly, niobium has only been used in the capacitor industry to avery minor extent, predominantly in areas of with lower qualityrequirements.

However, niobium oxide dielectric constant is about 1,5 times as high asthat of a similar tantalum oxide layer, which should allow in principle,for higher capacitance of niobium capacitors, subject to considerationsof stability and other factors.

As for tantalum itself, despite the success of the K₂TaF₇/sodiumreduction process, there are several drawbacks to this method.

It is a batch process subject to the inherent variability in the system;as a result, batch to batch consistency is difficult. Post reductionprocessing (mechanical and hydro-metallurgical separations, filtering)is complex, requiring considerable human and capital resources and it istime consuming. The disposal of large quantities of reaction productscontaining fluorides and chlorides can be a problem. Of fundamentalsignificance, the process has evolved to a state of maturity such thatthe prospects for significant advances in the performance of thetantalum powder produced are limited.

Over the years, numerous attempts were made to develop alternate waysfor reducing tantalum and similar metal compounds, includingNb-compounds, to the metallic state (Miller, G. L. “Tantalum andNiobium,” London, 1959, pp. 188-94; Marden, J. W. and Rich, M. H., U.S.Pat. No. 1,728,941, 1927; and Gardner, D., U.S. Pat. No. 2,516,863 1946;Hurd, U.S. Pat. No. 4,687,632). Among these were the use of activemetals other than sodium, such as calcium, magnesium and aluminum andraw materials such as tantalum pentoxide and tantalum chloride. As seenin Table I, below, the negative Gibbs free energy changes indicate thatthe reduction of the oxides of Ta, Nb and other metals with magnesium tothe metallic state is favorable; reaction rate and method determine thefeasibility of using this approach to produce high quality powders on acommercial scale. To date, none of these approaches were commercializedsignificantly because they did not produce high quality powders.Apparently, the reason these approaches failed in the past was becausethe reductions were carried out by blending the reducing agents with themetal oxide. The reaction took place in contact with the molten reducingagent and under conditions of inability to control the temperature ofhighly exothermic reactions. Therefore, one is unable to controlmorphology of the products and residual reducing metal content.

TABLE 1 Gibbs Free Energy Change for Reduction of Metal Oxides withMagnesium M_(x)O_(y)(s) + yMg(g) → yMgO(s) + xM(s) Temperature GibbsFree Energy Change (Kcal/mole oxide) ° C. Ta₂O₅ Nb₂O₅ TiO₂ V₂O₃ ZrO₂ WO₂200 −219 −254 −58 −133 −22 −143 400 −215 −249 −56 −130 −21 −141 600 −210−244 −55 −126 −20 −139 800 −202 −237 −52 −122 −18 −137 1000  −195 −229−50 −116 −15 −134 1200  −186 −221 −47 −111 −13 −131 1400  −178 −212 −45−106 −11 −128

The use of magnesium to deoxidize or reduce the oxygen content oftantalum metal is well known. The process involves blending the metalpowder with 1-3 percent magnesium and heating to achieve the reductionprocess. The magnesium is in the molten state during a portion of theheating time. In this case, the objective is to re-move 1000-3000 ppmoxygen and only a low concentration of MgO is produced. However, when amuch greater quantity of tantalum oxide is reduced a large quantity ofmagnesium oxide is generated. The resulting mixture of magnesium,tantalum oxide and magnesium oxide can under conditions of poorlycontrolled temperature, form tantalum-magnesium-oxygen complexes thatare difficult to separate from the tantalum metal.

It is a principal object of the invention to provide a new approach toproduction of high performance, capacitor grade tantalum and niobiumpowders that provides a means of eliminating one or more, preferablyall, the problems of traditional double salt reduction and follow onprocessing.

It is further object of the invention to enable a continuous productionprocess.

It is a further object of the invention to provide improved metal forms.

Another object is to provide niobium/tantalum alloy powders of capacitorgrade quality and morphology.

SUMMARY OF THE INVENTION

We have discovered that the prior art problems can be eliminated whenmetal oxides such as Ta₂O₅ and Nb₂O₅ and suboxides in massive amountsare reduced with magnesium in gaseous form, substantially or preferablyentirely. The oxide source should be substantially or preferablyentirely in solid. The oxide is provided in the form of a porous solidwith high access throughout its mass by the gaseous reducing agent.

The metals that can be effectively produced singly or in multiples(co-produced) through the present invention are in the group of Ta, Nb,and Ta/Nb alloy, any of these alone or with further inclusion of addedor co-produced Ti, Mo, V, W, Hf and/or Zr. The metals can also be mixedor alloyed during or after production and/or formed into usefulcompounds of such metals. The respective stable and unstable oxide formsof these metals can be used as sources. Metal alloys may be producedfrom alloyed oxide precursors, e.g. resulting from coprecipitation of asuitable precursor for the oxide.

Vapor pressures of some of the reducing agents are given as follows:

Temperature (° C.) Aluminum P (Atmospheres) 2,000 53. × 10⁻² 2,100 1.0 ×10⁻¹ 2,200 1.9 × 10⁻¹ 2,300 3.3 × 10⁻¹ 2,400 5.6 × 10⁻¹ 2,500 9.0 × 10⁻¹2,600 1.4 Temperature (° C.) Magnesium P (Atmospheres)  800 4.7 × 10⁻² 850 8.9 × 10⁻²  900 1.6 × 10⁻¹  950 2.7 × 10⁻¹ 1000 4.8 × 10⁻¹ 1050 7.2× 10⁻¹ 1100 1.1 Temperature (° C.) Calcium P (Atmospheres) 1,000 1.7 ×10⁻² 1,100 5.1 × 10⁻² 1,200 1.3 × 10⁻¹ 1,300 2.9 × 10⁻¹ 1,400 6.0 × 10⁻¹1,500 1.1 Temperature (° C.) Lithium P (Atmospheres) 1,000 5.1 × 10⁻²1,100 1.4 × 10⁻¹ 1,200 3.8 × 10⁻¹ 1,300 7.2 × 10⁻¹ 1,400 1.4

The temperature of reduction varies significantly depending on thereducing agent used. The temperature ranges for reduction of (Ta, Nb)oxide are: with Mg_((gas))−800-1,100° C., Al_((gas))−1,100-1,500° C.,Li_((gas))−1,000-1,400° C., Ba_((gas))−1,300-1,900° C.

Different physical properties as well as morphology of the metal powderproduced by reduction can be achieved by variations of temperature andother conditions of processing within the effective reduction range.

One embodiment of the invention includes a first step of reducing anoxide source of selected metal(s) substantially to free 80-100% (byweight) of the metal values therein as primary powder particles, thenleaching or other steps of hydrometallurgy to separate the metal fromresidual reducing agent oxide and other byproducts of the reductionreaction and from residual condensed reducing agent (optionally),followed by one or more deoxidation steps under less concentratedreagent conditions than in the first gross reduction step (and withbetter tolerance of molten state of the reducing agent), then furtherseparation as might be needed.

In accordance with this first embodiment the invention provides for asingle stage reduction process for the production of metal powders ascited above, comprising the steps of:

(a) providing an oxide or mixed oxides of the metal(s), the oxide itselfbeing in a form that is traversable by gas,

(b) generating a gaseous reducing agent at a site outside the oxide massand passing the gas through the mass at an elevated temperature,

(c) the reactants selection, porosity of the oxide, temperature and timeof the reduction reaction being selected for substantially completereduction of the oxide(s) to free the metal portion thereof, theresidual oxide of reducing agent formed in the reaction being easilyremovable,

whereby a high surface area, flowable metal powder is formed in aprocess that essentially avoids use of molten state reducing agent inproduction of metal or alloy powder.

Preferred reducing agents used in this reduction process of the firstembodiment are Mg, Ca and/or their hydrides. Particularly preferred isMg.

Preferred is the production of Nb and/or Ta metals, optionally alloyedwith each other and/or with alloying elements, selected from the groupconsisting of Ti, Mo, W, Hf, V and Zr.

A second embodiment of the invention provides for a two-stage reductionprocess, comprising the steps of:

(a) providing an oxide or mixed oxide of the metal(s), the oxide beingin a form that is traversible by gas,

(b) passing a hydrogen containing gas, alone or with gaseous diluent,through the mass at an elevated temperature in a manner for partialreduction of the oxide(s),

(c) the porosity of the oxide, temperature and time of reductionreaction being selected to remove at least 20% of the oxygen containedin the oxide to produce a suboxide,

(d) reducing the suboxide with reducing metal(s) and/or hydrides of oneor more reducing metals, thereby substantially completely reducing theoxide to free the metal portion thereof.

Preferrably the reducing metals and/or metal hydrides are brought intocontact with the suboxide in gaseous form.

Preferred reducing metals in the second reduction step of this secondembodiment are Mg and/or Ca and/or their hydrides. Particularlypreferred is Mg.

Reduction temperature preferably (for Mg) is selected between 850° C. upto normal boiling point (1150° C.)

The process according to the present invention (both embodiments)specifically has been developed to provide capacitor grade tantalum andniobium and tantalum nio-bium alloy powders and Ta/Nb materials orapplication of equivalent purity and/or morphology needs. The greatestgap of the state of the art is filled in part by the availability ofcapacitor grade niobium enabled by this invention, but a segment of thetantalum art is also enhanced thereby. In all cases the tantalum and/orniobium may be enhanced by alloying or compounding with other materialsduring the reduction reaction production of the tantalum/niobium orthereafter. Among the requirements for such powders is the need for ahigh specific surface presintered agglomerate structure of approximatelyspherical primary particles which after pressing and sintering resultsin a coherent porous mass providing an interconnected system of porechannels with gradually narrowing diameter to allow easy entrance of theforming electrolyte for anodization and manganese nitrate solution[Mn(NO₃)₂] for manganization.

The reduction of oxides with gaseous reducing agents at least during theinitial reduction phase allows for easy control of temperature duringreduction to avoid excessive presintering. Furthermore, as compared toprior art proposals using liquid reducing metals, the controlledreduction with gaseous reducing metals does not lead to contamination ofthe reduced metal with the reducing metal by incorporation into thereduced metal lattice. It has been found that such contamination mainlyoccurs during the initial reduction of (in case of Nb) Nb₂O₅ to NbO₂.This at first appeared surprising because niobium suboxide (NbO₂)contains only 20% less oxygen than niobium pentoxide (NbO_(2.5)). Thiseffect was traced back to the fact that the suboxide forms aconsiderably more dense crystal lattice than the pentoxide. The densityof NbO_(2.5) is 4.47 g/cm³, whilst that of NbO₂ is 7.28 g/cm³, i.e., thedensity is increased by 1.6 times by the removal of only 20% of theoxygen. Taking into account the different atomic weights of niobium andoxygen, a reduction in volume of 42% is associated with the reduction ofNbO_(2.5) to NbO₂. Accordingly, Applicants state (without limiting thescope of the invention thereby) that the effect according to theinvention can be explained in that during the reduction of the pentoxidemagnesium in contact with the oxide is able to diffuse relatively easilyinto the lattice, where it has a high mobility, whereas the mobility ofmagnesium in the suboxide lattice is significantly reduced. Accordingly,during the reduction of the suboxide the magnesium substantially remainson the surface and remains accessible to attack by washing acids.

This even applies in case of a controlled reduction with gaseousmagnesium. Obviously in this case reduction occurs also during thecritical initial reduction to suboxide only at the surface of the oxide,and magnesium oxide formed during reduction does not enter the oxide orsuboxide powder. Preferred temperature during reduction with magnesiumgas is between 900 and 1100° C., particularly preferred between 900 and1000° C.

Temperature may be increased up to 1200° C. after at least 20% of theoxygen is removed to improve presintering.

The reduction of the pentoxide with hydrogen produces a suboxide whichis already sintered with the formation of agglomerates comprising stablesintered bridges, which have a favorable structure for use as acapacitor material.

Lower temperatures necessitate longer times of reduction. Moreover, thedegree of sintering of the metal powders to be produced can be adjustedin a predeterminable manner by the choice of reduction temperature andreduction time. The reactors are preferably lined with molybdenum sheetor by a ceramic which is not reduced by H₂, in order to preventcontamination.

Furthermore, the reduction time and reduction temperature should beselected so that at least 20% of the oxygen is removed from thepentoxide. Higher degrees of reduction are not harmful. However, it isgenerally not possible to reduce more than 60% of the oxygen withinpracticable time scales and at tolerable temperature.

After a degree of reduction of 20% or more has been reached, thesuboxide is present. According to this process embodiment the reductionproduct is preferably still held (annealed) for some time, mostpreferably for about 60 to 360 minutes, at a temperature above 1000° C.It appears that this enables that the new, dense, crystal structure canbe formed and stabilized. Since the rate of reduction decreases veryconsiderably with the degree of reduction, it is sufficient to heat thesuboxide at the reduction temperature under hydrogen, optionally with aslight decrease in temperature. Reduction and annealing times of 2 to 6hours within the temperature range from 1100 to 1500° C. are typicallysufficient. Moreover, reduction with hydrogen has the advantage thatimpurities such as F, Cl and C, which are critical for capacitorapplications, are reduced to less than 10 ppm, preferably less than 2ppm.

The suboxide is subsequently cooled to room temperature (<100° C.) inthe reduction apparatus, the suboxide powder is mixed with finelydivided powders of the reducing metals or metal hydrides and the mixtureis heated under an inert gas to the reduction temperature of the secondstage. The reducing metals or metal hydrides are preferably used in astoichiometric amount with respect to residual oxygen of the acid earthmetal suboxide, and are most preferably used in an amount which isslightly in excess of the stoichiometric amount.

One particularly preferred procedure consists of using an agitated bedin the first stage and of carrying out the second stage, withoutintermediate cooling, in the same reactor by introducing the reducingmetals or metal hydrides. If magnesium is used as the reducing metal,the magnesium is preferably introduced as magnesium gas, since in thismanner the reaction to form metal powder can readily be controlled.

After the reduction whether according to the one-stage or to thetwo-stage reduction process to metal is complete, the metal is cooled,and the inert gas is subsequently passed through the reactor with agradually increasing content of oxygen in order to deactivate the metalpowder. The oxides of the reducing metals are removed in the mannerknown in the art by washing with acids.

Tantalum and niobium pentoxides are preferably used in the form offinely divided powders. The primary grain size of the pentoxide powdersshould approximately correspond to 2 to 3 times the desired primarygrain size of the metal powders to be produced. The pentoxide particlespreferably consist of free-flowing agglomerates with average particlesizes of 20 to 1000 μm, including a specific preference of a narrowerrange of most preferably 50 to 300 μm particle size.

Reduction of niobium oxide with gaseous reducing agents can be conductedin an agitated or static bed, such as a rotary kiln, a fluidized bed, arack kiln, or in a sliding batt kiln. If a static bed is used, the beddepth should not exceed 5 to 15 cm, so that the reducing gas canpenetrate the bed. Greater bed depths are possible if a bed packing isemployed through which the gas flows from below. For tantalum, preferredequipment choices are described in Example 2 and the paragraph betweenExamples 2 and 3, below, with reference to FIGS. 1-4.

Niobium powders which are particularly preferred according to theinvention are obtained in the form of agglomerated primary particleswith a primary particle size of 100 to 1000 nm, wherein the agglomerateshave a particle size distribution as determined by Mastersizer(ASTM-B822) corresponding to D10=3 to 80 μm, particularly preferred 3 to7 μm, D50=20 to 250 μm, particularly preferred 70 to 250 μm, mostpreferably 130 to 180 μm and D90 =30 to 400, particularly preferred 230to 400 μm, most preferably 280 to 350 μm. The powders according to theinvention exhibit outstanding flow properties and pressed strengths,which determine their processability to produce capacitors. Theagglomerates are characterized by stable sintered bridges, which ensurea favorable porosity after processing to form capacitors.

Preferably niobium powder according to the invention contains oxygen inamounts of 2500 to 4500 ppm/m² surface and is otherwise low in oxygen,up to 10,000 ppm nitrogen and up to 150 ppm carbon, and without takinginto account a content of alloying metals has a maximum content of 350ppm of other metals, wherein the metal content is mainly that of thereducing metal or of the hydrogenation catalyst metal. The total contentof other metals amounts to not more than 100 ppm. The total content ofF, Cl, S is less than 10 ppm.

Capacitors can be produced from the niobium powders which are preferredaccording to the invention, immediately after deactivation and sievingthrough a sieve of mesh size 400 μm. After sintering at a presseddensity of 3,5 g/cm³ at 1100° C. and forming at 40 V these capacitorshave a specific capacitance of 80,000 to 250,000 μFV/g (as measured inphosphoric acid) and a specific leakage current density of less than 2nA/μFV. After sintering at 1150° C. and forming at 40 V, the specificcapacitor capacitance is 40,000 to 150,000 μFV/g with a specific leakagecurrent density of less than 1 nA/μFV. After sintering at 1250° C. andforming at 40 V, capacitors are obtained which have a specific capacitorcapacitance (as measured in phosphoric acid) of 30,000 to 80,000 μFV/gand a specific leakage current density of less than 1 nA/μFV.

The niobium powders which are preferred according to the invention havea BET specific surface of 1.5 to 30 m²/g, preferably of 2 to 10 m²/g.

Surprisingly it has been found that capacitors can be made fromNb/Ta-alloy powders in way that the capacitors have an appreciablyhigher specific capacitance obtained from capacitors made from pureNb-and pure Ta-powers or anticipated for an alloy be simple linearinterpolation. Capacitances (μFV) of capacitors with sintered Nb-powderanodes and sintered Ta-powder anodes having the same surface area areapproximately equal. The reason is that the higher dielectric constantof the insulating niobium oxide layer (41 as compared to 26 of tantalumoxide) is compensated by the larger thickness of the oxide layer pervolt (anodization voltage) formed during anodization. The oxide layerthickness per volt of Nb is about twice as thick as that formed on Ta(about 1.8 nm/V in the case of Ta and about 3.75 nm/V in the case ofNb). The present invention can provide a surface related capacitance(μFV/m²) of alloy powder capacitors which is up to about 1.5 to 1.7higher than the expected value from linear interpolation between Nbpowder capacitors and Ta powder capacitors. This seems to indicate thatoxide layer thickness per volt of anodization voltage of alloy powdersof the invention is closer to that of Ta, whereas the dielectricconstant of the oxide layer is closer to that of Nb. The foregoingsurprisingly high capacitance of the alloy may be associated with adifferent structural form of oxide of alloy components compared tostructure of oxides on surfaces of pure Nb powders. Indeed, preliminarymeasurements have revealed that oxide layer growth of a 15 at.-%Ta—85at.-% Nb alloy is almost 2.75 nm/volt.

The present invention accordingly further comprises an alloy powder foruse in the manufacture of electrolyte capacitors consisting primarily ofniobium and containing up to 40 at.-% of tantalum based on the totalcontent of Nb and Ta. Alloy powder in accordance with the presentinvention shall mean that the minor Ta-component shall be present in anamount greater than the amount of ordinary impurity of niobium metal,e.g. in an amount of more than 0.2% by weight (2000 ppm, correspondingto 2 at.-% for Ta).

Preferrably, the content of Ta is at least 2 at.-% of tantalum,particularly preferred at least 5 at.-% of tantalum, most preferably atleast 12 at.-% of tantalum, based on the total content of Nb and Ta.

Preferably the content of tantalum in the alloy powders in accordancewith the invention is less than 34 at.-% of tantalum. The effect ofcapacitance increase is gradually increasing up to a ratio of Nb- toTa-atoms of about 3. Higher than 25 at.-% Ta based on the total contentof Nb and Ta does only slightly further increase the effect.

The alloy powders according to the invention preferably haveBET-surfaces multiplied with the alloy density of between 8 and 250(m²/g)×(g/cm³), particularly preferred between 15 and 80 (m²/g)×(g/cm³).The density of the alloy material may be calculated from the respectiveatomic ratio of Nb and Ta multiplied by the densities of Nb and Tarespectively.

The effect of capacitance increase of alloying is not limited to powdershaving the structure of agglomerated spherical grains. Accordingly thealloyed powders in accordance of the invention may have a morphology inthe form agglomerated flakes preferably having have a BET-surface timesdensity of between 8 and 45 (m²/g)×(g/cm³).

Particularly preferred alloy powders are agglomerates of substantiallyspherical primary particles having a BET-surface times density of 15 to60 (m²/g)×(g/cm³). The primary alloy powders (grains) may have meandiameters of between 100 to 1500 nm, preferrably 100 to 300 nm.Preferrably the deviation of diameter of primary particles from meandiameter is less than a factor 2 in both directions.

The agglomerate powders may have a mean particle size as determined inaccordance with ASTM-B 822 (Mastersizer) as disclosed for niobiumpowders above.

Particularly preferred alloy powders have a ratio of Scott density andalloy density of between 1.5 and 3 (g/inch³)/(g/cm³).

Any production method known in the art for the production of capacitorgrade tantalum powder may be used, provided that a precursor is usedwhich is an alloyed precursor containing niobium and tantalumapproximately at the atomic ratio of Nb and Ta desired in the metalpowder alloy instead of precursor containing tantalum alone.

Useful alloy precursors may be obtained from coprecipitation of(Nb,Ta)-compounds from aqueous solutions containing water soluble Nb-and Ta-compounds e.g. coprecipitation of (Nb, Ta)-oxyhydrate fromaqueous solution of heptafluoro-complexes by the addition of ammonia andsubsequent calcination of the oxhydrate to oxide.

Flaked powders may be obtained by electron beam melting of a blend ofhigh purity tantalum and niobium oxides, reducing the molten ingot,hydriding the ingot at elevated temperature, and comminuting the brittlealloy, dehydriding the alloy powder and forming it into flakes. Theflakes are thereafter agglomerated by heating to 1100 to 1400° C. in thepresence of a reducing metal such as Mg, optionally with doping with Pand/or N. This process for the manufacture of “ingot derived” powder isgenerally known from U.S. Pat. No. 4,740,238 for the production oftantalum flaked powder and from WO 98/19811 for niobium flaked powder.

Particularly preferred Nb-Ta-alloy powders having the morphology ofagglomerated spherical grains are produced from mixed (Nb, Ta)-oxides byreduction with gaseous reducing agent as described herein.

The metal powders produced are suitable for use in electronic capacitorsand other applications including, e.g. the production of complexelectro-optical, superconductive and other metal and ceramic compounds,such as PMN structures and high temperatures form metals and oxide.

The invention comprises the said powders, the methods of producing suchpowders, certain derivative products made from such powders and methodsfor making such derivative products.

The capacitor usage can be accompanied by other known artifacts ofcapacitor production such as doping with agents to retard sinterdensification or otherwise enhance end product capacitance, leakage andvoltage breakdown.

The invention enables several distinct breakthroughs in several of itsvarious fields of application.

First, the well known high performance tantalum powders for makingcomputer/telecommunications grade solid electrolyte, small sizecapacitors (high capacitance per unit volume and stable performancecharacteristics) can now be made with substantial net savings of cost,complexity and time.

Second, other reactive metals—especially Nb and alloys, e.g. Ta—Nb,Ta—Ti, Nb—Ti, can be introduced as replacement for Ta in capacitors incertain applications with a cost saving or as replacement for the highend Al market with much better performance, particularly enabling muchsmaller sizes for equivalent capacitance and use of solid electrolyte.Commercial aluminum electrolytic capacitors use wet electrolyte systems.

Other objects, features and advantages will be apparent from thefollowing detailed description of preferred embodiments taken inconjunction with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1-4 show sketch outlines of processing systems for practice of thepresent invention;

FIGS. 5A-12C are scanning electron micrographs (SEMs) of powdersproduced according to the present invention, including some SEMs ofstate of the art or comparison examples of metal powders made otherwisethan in accordance with the present invention;

FIGS. 13 and 14 are flow charts illustrating diverse usages of thepowder and derivatives; and

FIG. 15 is a schematic representation of an end item according to usageas a capacitor (one of several forms of capacitor usage).

FIG. 16 is a trace of capacitance and surface area of Ta—Nb alloypowders in relation to alloy composition.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS EXAMPLE 1 (COMPARISON)

A mixture of Ta₂O₅ and magnesium was loaded into a tantalum tray andcovered with tantalum foil. The magnesium stoichiometry was 109% of thatrequired to completely reduce the tantalum oxide. The mixture was heatedat 1000° for six hours in an argon atmosphere. The mixture was notagitated during the reduction process. After cooling, the products werepassivated by programmed addition of oxygen. The result of the reductionprocess was a black spongy material that was difficult to break up. Theproduct was leached with dilute mineral acid to remove the magnesiumoxide, dried and screened. The yield of the coarse (+40 mesh) materialwas high at 25 percent. The impurity content of each (as % or ppm) andsurface areas (SA, cm²/gm) of the +40 and −40 fractions are given inTable 1.1, below. Both the magnesium and oxygen contents were high. Thelarge percentage of coarse material and poor quality of the product madeit unsuitable for use in capacitor applications.

TABLE 1.1 O N C S Na K Mg Sa % ppm ppm ppm ppm ppm ppm cm²/gm +40 mesh7.6 840 21 <5 <1 <10 >7000 17,000 −40 mesh 4.7 413 57 <5 <5 <10 >700035,000

EXAMPLE 2

Referring to FIG. 1, a bed (3) of 200 grams of tantalum pentoxide w asplaced on a porous tantalum plate 4 suspended above magnesium metalchips (5) contained in a tantalum boat. The container was covered with atantalum lid and placed in a sealed retort with argon (Ar) passedthrough the sealed volume via nozzle (6). The boat was heated to andmaintained at 1000° C. for six hours in an argon/magnesium gasatmosphere utilizing a bed (5) of solid magnesium chips maintained in aregion wholly separate from the oxide bed. After cooling to roomtemperature, the product mixture was passivated by introducingargon-oxygen mixtures, containing 2, 4, 8, 15 inches (Hg, partialpressure) of O₂ (g), respectively, into the furnace. Each mixture was incontact with powder for 30 minutes. The hold time for the lastpassivation with air was 60 minutes.

The magnesium oxide was separated from the tantalum powder by leachingwith dilute sulfuric acid and then rinsed with high purity water toremove acid residues. The product was a free flowing, powder. Samples ofthe product (designated as Ta GR-2D) are shown in scanning electronmicrographs (SEMs) at FIGS. 5A, 5B, 5C at 15,700, 30,900 and 60,300magnifications, respectively, taken in an electron microscope operatedat 15 kilovolts. A comparison is given in FIGS. 5D and 5E which are70,000 magnification (×) SEMs of tantalum powder made by sodiumreduction. Properties of the tantalum powder of FIGS. 5A, 5B, 5C aregiven in Table 2.1, below.

TABLE 2.1 Surface Content of Included Chemical Elements (ppm) area O N CCr Fe Ni Na K Ca Si (cm²/gm) 12,900 126 75 <5 23 <5 <1 <10 <2 <8 37,600

The oxygen concentration to surface area ratio was consistent withsurface oxygen only, indicating that the tantalum oxide was completelyreduced.

Alternate forms of reactor to the one shown in FIG. 1 (and discussed inExample 2) are shown in FIGS. 2-4. FIG. 2 shows a flash reactor 20 witha vertical tube surrounded by a heater 24, a feed source 25 of metaloxide and a source 26 of reducing agent (e.g. Mg) vapor (mixed inargon), an argon outlet 26′ and a collector 28 for metal and oxide ofthe reducing agent. Valves V1, V2 are provided. Particles of the oxidedrop through the tube and are flash reduced. FIG. 3 shows a rotary kiln30 with an inclined rotating tube 32, heater 34, oxide hopper 35, gassource (reducing agent and diluent, e.g. argon) and outlet 36, 36′, andcollector 38 for metal and reducing agent oxide. FIG. 4 shows a multiplehearth furnace 40 with a retort 42 containing rotary trays 43 andsplined paddles, 43 and splined paddles, 43, heater 44, oxide source 45,gas source and exit 46, 46′ and collector 48. Still other forms ofreactors such as conventional per se fluid bed furnace reactors orContop, KIVCET types can be used.

EXAMPLE 3

Tantalum powder with surface area of 57,000 cm²/gm made according to theprocedure in Example 2 was deoxidized by blending the powder with 2 W/W% Mg and heating at 850° C. for two hours in an argon atmosphere.Separation of reducing agent source and oxide is not necessary in thisfollow up deoxidation step. The deoxidized powder was allowed to cooland then passivated, leached, and dried. A SEM (100,000×) of thedeoxidized (finished) powder appears at FIG. 7A and a SEM (70,000×) offinished sodium reduced powders appears at FIG. 7B. the morphologydifferences are apparent. After doping with 100 ppm P by adding anappropriate amount of NH₄H₂PO₄, the powder was pressed into pelletsweighing 0.14 grams at a press density of 5.0 g/cc. A SEM of the furtherdeoxidized powder is given at FIG. 6. The pellets were sintered invacuum at 1200° C. for 20 minutes. The pellets were anodized to 30 voltsin 0.1 volume percent (V/V %) H₃PO₄ solution at 80° C. The formationcurrent density was 100 mA/gm and the hold time at the formation voltagewas two hours. The average capacitance of the anodized pellets was 105,000 μF (V)/gm and the leakage current measured after five minutesapplication of 21V was 0.1 nA/μF (V).

EXAMPLE 4

Powder with surface area of 133,000 cm²/gm and bulk density of 27.3 g/m³made as described in Example 2 was treated as in Example 3. A SEM(56,600×) of the finished powder appears at FIG. 7C. Pellets made fromthe deoxidized powder were anodized to 16V using the conditions inExample 3. The average capacitance of the anodized pellets was 160,000μF (V)/gm.

EXAMPLE 5

Nine hundred grams of Ta₂O₅ was reduced with gaseous magnesium at 900°C. for two hours. The magnesium oxide was removed from the reductionproduct by leaching with dilute sulfuric acid. The resulting powder hada surface area of 70,000 cm²/gm and was deoxidized at 850° C. for twohours using 8 W/W % magnesium. One (1.0) W/W % NH₄Cl was added to thecharge to nitride the tantalum. The deoxidized powder was treated asdescribed in Example 3. The P doping level was 200 ppm. The powder wasdeoxidized again using the same time and temperature profile with 2.0W/W % Mg and no NH₄Cl. Residual magnesium and magnesium oxide wereremoved by leaching with dilute mineral acid. The chemical properties ofthe powder are given in Table 5.1, below. The powder had a surface areaof 9,000 cm²/gm and excellent flowability. Pressed pellets were sinteredat 1,350° C. for twenty minutes and anodized to 16V in 0.1 V/V % H₃PO₄at 80° C.

The capacitance of the anodized pellets was 27,500 μF (V)/gm and theleakage was 0.43 nA/μF (V).

TABLE 5.1 Chemical Element (ppm) O N C Cr Fe Ni Na K Ca Si 2610 2640 958 18 <5 1 <10 <2 41

EXAMPLE 6

500 grams of Ta₂O₅ were reduced at 1,000° C. for six hours with gaseousmagnesium. Properties of the primary powder so produced are given inTable 6.1, below:

TABLE 6.1 O, ppm N, ppm C, ppm Na, ppm K, ppm SA, cm²/g 19,000 1693 49<1 <10 60,600

The primary powder was deoxidized at 850° C. for two hours. 4 W/W % Mgand 1 W/W % NH₄Cl were added. MgO was leached with mineral acid. Thenthe powder was doped at 200 ppm P by adding the equivalent amount ofNH₄H₂PO₄. The powder was deoxidized for the second time at 850° C. fortwo hours and then nitrided at 325° C. by adding a gaseous mixturecontaining 80% argon and 20% nitrogen. Some properties of the finishedpowder are given in Table 6.2, below.

TABLE 6.2 O-ppm N, ppm C, ppm Na, ppm K, ppm SA, cm²/g 6050 3430 54 <1<10 24,300

Pellets were made from the powder at a press density of 5.0 gm/cc. Thesintered pellets were anodized at 80° C. to 16 volts in 0.1 W/W % H₃PO₄solution. Capacitances and leakages as a function of sinteringtemperature are given in Table 6.3, below.

TABLE 6.3 Sintering Capacitance Leakage Temperature (° C.) μF (V)/gmμA/μ F(V) 1,200 143,000 0.77 1,250 121,000 0.88 1,300 96,000 1.01

EXAMPLE 7 (COMPARATIVE)

Potassium heptafluoroniobate (K₂NbF₇) was reduced with sodium using astirred reactor molten salt process similar to the ones described byHellier et al. and Hildreth et al., U.S. Pat. No. 5,442,978. The diluentsalt was sodium chloride and the reactor was made from Inconel alloy.The niobium metal powder was separated from the salt matrix by leachingwith dilute nitric acid (HNO₃) and then rinsing with water. Selectedphysical and chemical properties are given in Table 7.1, below. The veryhigh concentrations of the metallic elements, nickel, iron and chromium,make the powders unsuitable for use as capacitor grade material. Thecontamination resulted because of the inherent corrosive nature of theK₂NbF₇. This property makes the sodium reduction process unsuitable formaking capacitor grade niobium powder.

TABLE 7.1 Sample SA SBD FAPD O (ppm) Ni Cr Fe 1 13820 8.7 1.76 608018000 2970 2660 2 11700 9.4 1.48 4930 11300 4790 2060

SBD=Scott Bulk Density (g/in³), FAPD=Fisher Average Particle Diameter(μ)

EXAMPLE 8

Two hundred grams of niobium pentoxide was reduced as described inExample 2. The resulting product was a free flowing black powder and hada surface area of 200,800 cm²/gm. The passivated product was leachedwith dilute nitric acid solution to remove magnesium oxide and residualmagnesium and then with high purity water to remove residual acid. Thismaterial was blended with ten (10.0) W/W % Mg and deoxidized at 850° C.for two hours. Physical and chemical properties of the powder are listedin table 8.1, below. The powder was doped with 100 ppm P as described inExample 3.

TABLE 8.1 Physical and Chemical Properties of Niobium Powder SurfaceChemical Element (ppm) Area O N C Cr Fe Ni Na K Ca Si cm²/gm 13000 62040 27 45 21 8 1 3 26 40,900

SEMs (70,000×) appear at FIGS. 8A and 8B, respectively, for niobiumpowders produced by liquid sodium (Ex. 7) and magnesium gas (Ex. 8)reduction. Note the clustering of small particles as barnacles on largeones is much more pronounced in FIG. 8B than in 8A. FIGS. 8C, 8D areSEMs (2,000×) of, respectively niobium powder as produced by sodiumreduction and magnesium gas reduction.

The niobium powder produced by liquid sodium reduction has large (>700nm) joined (300 nm+) grains protruding and facets that give the producta blocky shape and fine grain material (order of 10 nm, but some up to75 nm) as barnacles while the niobium powder produced by magnesium gasreduction has a base grain size of about 400 nm and many smaller grainsof about 20 nm thereon many of which smaller grains are themselvesagglomerates of up to 100 nm in size.

EXAMPLE 9

Pellets weighing 0.14 gm were prepared from the niobium powder producedin Example 8. The pellets were anodized in 0.1 V/V % H₃PO₄ solution at80° C. The current density was 100 mA/gm and the hold time at theformation voltage was two hours. Electrical results as a function ofpellet press density, formation voltage and sintering temperature aregiven in Table 9.1, below.

TABLE 9.1 Summary of Electrical Properties (capacitance, leakage) ofNiobium Powder at 3.0, 3.5 (g/cc) Press Densities Sintering TemperatureCapacitance Leakage (° C.) (μF (V)/gm) (nA/μF(V)) 3.0 3.5 3.0 3.5 16 VFormation 1300 29,500 20,000 1.6 4.7 1350 21,000 16,000 0.7 1.5 40 VFormation 1250 53,200 44,500 2.1 4.0 1300 31,000 22,300 1.2 4.7 135026,500 20,000 0.7 1.0

EXAMPLE 10

Niobium oxide was reduced with gaseous magnesium as described in Example8. The resulting powder was deoxidized twice. During the firstdeoxidation, 2.0 W/W % NH₄Cl was added to the charge to nitride thepowder. The deoxidation conditions were 850° C. for two hours with 7.0W/W % Mg. After leaching and drying, the powder was doped with 200 ppmP. The second deoxidation was carried out at 850° C. for two hours using2.5 W/W % Mg. The finished powder has a surface area of 22,000 cm²/gmand good flowability. The chemical properties are given in Table 10.1,below. Pellets were anodized to 16 volts in 0.1 VN % H₃PO₄ solution at80° C. using a current density of 100 mA/g and a two-hour hold. Theelectrical properties are given in Table 10.2, below.

TABLE 10.1 Chemical Element (ppm) O N C S Cr Fe Ni Si Ta 7490 8600 166 9<20 114 <20 34 <200

TABLE 10.2 Electrical Properties Sintering Capacitance LeakageTemperature (° C.) (μF(V)/gm (nA/μF(V) 1250 68,000 0.24 1300 34,500 0.141350 11,300 0.32

EXAMPLE 11

a) The Nb₂O₅ used had a particle size of 1.7 μm as determined by FSSS(Fisher Sub Sieve Sizer) and comprised the following contents ofimpurities:

Total (Na, K, Ca, Mg) 11 ppm Total (Al, Co, Cr, Cu, Fe, Ga, 19 ppm Mn,Mo, Ni, Pb, Sb, Sn, Ti, V, W, Zn, Zr) Ta 8 ppm Si 7 ppm C <1 ppm Cl <3ppm F 5 ppm S <1 ppm

The Nb₂O₅ was passed in a molybdenum boat through a sliding batt kiln,under a slowly flowing hydrogen atmosphere, and was maintained in thehot zone of the kiln for 3.5 hours.

The suboxide obtained had a composition corresponding to NbO₂.

b) The product was placed on a fine-mesh grid under which a crucible wassituated which contained magnesium in 1.1 times the stoichiometricamount with respect to the oxygen content of the suboxide.

The arrangement comprising the grid and crucible was treated for 6 hoursat 1000° C. under an argon protective gas. In the course of thisprocedure, the magnesium evaporated and reacted with the overlyingsuboxide. The kiln was subsequently cooled (<100° C.) and air wasgradually introduced in order to passivate the surface of the metalpowder.

The product was washed with sulfuric acid until magnesium could nolonger be detected in the filtrate, and thereafter was washed untilneutral with deionized water and dried.

Analysis of the niobium powder gave the following impurity contents:

O 20,000 ppm (3300 ppm/m²) Mg 200 ppm Fe 8 ppm Cr 13 ppm Ni 3 ppm Ta 110ppm C 19 ppm N 4150 ppm

The particle size distribution, as determined by Mastersizer,corresponded to

D10 4.27 μm

D50 160.90 μm

D90 318.33 μm

The primary grain size was determined visually as about 500 nm. TheScott bulk density was 15.5 g/inch³. The BET specific surface was 6.08m²/g. The flowability, determined as the Hall Flow, was 38 seconds.

c) Anodes with a diameter of 3 mm, a length of 5.66 mm, an anode mass of0.14 g and a pressed density of 3.5 g/cm³ were produced from the niobiumpowder by sintering on a niobium wire for the times and at thetemperatures given in Table 11.1.

The pressed strength of the anodes, as determined according toChatillon, was 6.37 kg. The anodes were formed at 80° C. in anelectrolyte containing 0.1% by volume of H₃PO₄ at a current density of100/150 mA at the voltage given in Table 11.1 and the capacitorcharacteristics were determined; see Table 11.1

TABLE 11.1 Sintering Temp./ Wire time Sintered drawing Forming Ca-Leakage ° C. density strength voltage pacitance current Sample min g/cm³N V μFV/g nA/μFV a 1250/20 5.1 16  41,126 0.47 b 5 40  41,725 0.7  c 570  23,721 2.13 d 1150/20 3.9 35.6 16 111,792 0.77 e 4 35.6 40 147,2920.43 f 1100/20 3.75 36.6 16 194,087 0.4  g 3.7 36.1 40 194,469 0.36

EXAMPLE 12

Example 11 was repeated, with the difference that the temperature in thefirst reduction stage was 1300° C.

The metal powder had the following properties:

Mastersizer

D10 69.67 μm

D50 183.57 μm

D90 294.5 μm

Primary grain size (visual) 300-400 nm

BET specific surface 5 m²/g

Free-flowing.

The pressed strength was extremely high:

13 kg at a pressed density of 3.5 g/cm³, and

8 kg at a pressed density of 3 g /cm³.

After sintering at 1100° C. for 20 minutes (pressed density 3 g/m³), andafter forming at 40 V, a capacitance of 222,498 μFV/g and a leakagecurrent of 0.19 nA/μFV were measured.

EXAMPLE 13

This example shows the effect of the reduction temperature in the firststage on the properties of the niobium powder:

Three batches of niobium pentoxide were treated for 4 hours underhydrogen at 1100° C., 1300° C. or 1500° C., under conditions which wereotherwise the same.

The batches were subsequently reduced to niobium metal with Mg gas (6hours, 1000° C.). The MgO which was formed in the course of thereaction, together with excess Mg, were washed out with sulfuric acid.The following powder properties were obtained:

Reduction temperature 1100° C. 1300° C. 1500° C. Suboxide: BET m²/g¹⁾1.03 0.49 0.16 Hall Flow²⁾ non- 25 g in 25 g in flowing 48 sec. 20 sec.Niobium metal: BET m²/g 9.93 7.8 5.23 FSSS μm³⁾ 0.6 0.7 6.8 Hall Flownon- 25 g in 25 g in flowing 85 sec. 19 sec. SD g/inch⁴⁾ 16.8 16.5 16.8Mg ppm 240 144 210 O ppm 40,000 28,100 16,600 ¹⁾BET specific surface²⁾flowability ³⁾particle size as determined by Fisher Sub Sieve Sizer⁴⁾bulk density

EXAMPLE 14

A (Nb_(x), Ta_(1−x))₂O₅ precursor is prepared by coprecipitation of (Nb,Ta)-oxyhydrate from mixed aqueous solution of niobium and tantalumheptafluorocomplexes by the addition of ammonia with stirring andsubsequent calcination of the oxyhydrate to oxide.

A lot of the mixed oxide powder having a nominal composition of Nb:Ta=90:10 (weight ratio) was placed in a molybdenum boat and passedthrough a sliding batt kiln under slowly flowing hydrogen atmosphere andwas maintained in the hot zone of the kiln for 4 hours at 1300° C. Aftercooling down to room temperature the composition was determined fromweight loss to be approximately (Nb_(0.944)Ta_(0.054))O.

The suboxide was placed on a fine mesh grid under which a crucible wassituated which contained magnesium in 1.2 times the stoichiometricamount with respect to the oxygen content of the suboxide. Thearrangement comprising grid and crucible was treated for 6 hours at1000° C. under an argon protective gas. The kiln was subsequently cooledto below 100° C. and air was gradually introduced in order to passivatethe surface of the metal powder.

The product was washed with sulfuric acid until magnesium could nolonger be detected in the filtrate, and thereafter washed until neutralwith deionized water and dried.

Analysis of the alloy powder gave a tantalum content of 9.73 wt.-% andthe following impurity contents (ppm):

O: 20500, Mg: 24, C: 39, Fe: 11, Cr: 19, Ni: 2, Mo: 100.

The primary grain size as determined visually was roughly 450 nm. BETspecific surface was 6.4 m²/g, Scott density 15.1 g/in³, particle size(FSSS) was 0.87 μm.

Anodes with a diameter of 2.94 mm, a length of 3.2 mm and a presseddensity of 3.23 g/cm³ were produced from the alloy powder by sinteringon a niobium wire for 20 minutes at 1150° C. Sintered density was 3.42g/cm³. The electrodes were anodized in an electrolyte containing 0.25%of H₃PO₄ until a final voltage of 40 V.

The capacitor characteristics were determined by using a 10% H₃PO₄aqueous solution as follows: Capacitance: 209117 μFV/g, Leakage current:0.55 nA/μFg.

EXAMPLE 15

An alloy powder was prepared as in Example 14, using an oxide powderwith nominal composition of Nb:Ta=75:25 (weight ratio).

Analysis of the metal alloy powder gave a tantalum content of 26,74wt.-% and the following impurity contents (ppm):

O: 15000, Mg: 25, C: 43, Fe: 9, Cr: 20, Ni: 2, Mo: 7, N: 247.

The primary grain size as determined visually was roughly 400 nm. BETspecific surface was 3.9 m²/g, Scott density 17.86 g/in³, particle size(FSSS) was 2.95 μm, Hall Flow 27.0 s.

Anodes with a diameter of 2.99 mm, a length of 3.23 mm and a presseddensity of 3.05 g/cm³ were produced from the alloy powder by sinteringon a niobium wire for 20 minutes at 1,150° C. Sintered density was 3.43g/cm³. The electrodes were anodized in an electrolyte containing 0.25%of H₃PO₄ until a final voltage of 40 V.

The capacitor characteristics were determined by using a 10% H₃PO₄aqueous solution as follows: Capacitance: 290173 μFV/g, Leakage current:0.44 nA/μFg.

EXAMPLE 16

Tantalum hydroxide was precipitated from an aqueous tantalumfluorocomplex solution by addition of ammonia. The precipitatedhydroxide was calcined at 1100° C. for 4 hours to provide a Ta₂O₅precursor with the following physical data: average particle diameterwith Fisher Sub Sieve Sizer (FSSS): 7.3 μm, bulk density (Scott): 27.8g/inch³, specific surface area (BET): 0.36 m²/g particle sizedistribution with laser diffraction on Master Sizer S, measured withoutultrasound: D10=15.07 μm, D50=23.65 μm, D90=34.03 μm.

The morphology of agglomerated spheres is shown on FIGS. 9A-9C(SEM-pictures).

300 g of the precursor pentoxide was placed on the screen and 124 g Mg(1.5 times the stoichiometric amount necessary to reduce the pentoxideto metal) was placed on the bottom of a retort shown in FIG. 1.

The retort was evacuated, filled with argon and heated to 950° C. for 12hours. After cooling to below 100° C., and passivation the product wasleached with an aqueous solution containing 23 wt.-% sulfuric acid and5.5 wt.-% hydrogen peroxide and thereafter washed with water untilneutral. The product was dried over night at 50° C. and screened <400μm.

The tantalum powder showed the following analytical data:

Average particle size (FSSS): 1.21 μm,

bulk density (Scott): 25.5 g/inch³,

BET surface: 2,20 m²/g,

good flowability,

MasterSizer D10=12.38 μm, D50=21.47 μm, D90=32.38 μm,

morphology: see FIGS. 10-10C (SEM-pictures).

Chemical analysis:

O: 7150 ppm

N: 488 ppm

H: 195 ppm

C: 50 ppm

Si: 30 ppm

F: 2 ppm

Mg: 6 ppm

Na: 1 ppm

Fe: 3 ppm

Cr: <2 ppm

Ni: <3 ppm.

The powder was soaked with gentle stirring with NH₄H₂PO₄-solutioncontaining 1 mg P per ml, dried over night at 50° C. for doping with 150ppm P and screened <400 μm.

Capacitor anodes were prepared from 0.047 g of Ta-powder each at presseddensity of 5.0 g/cm³ by sintering at 1260° C. with 10 minutes holdingtime.

Forming current density was 150 mA/g with 0,1 wt.-% H₃PO₄ solution asforming electrolyte at 85° C. until final voltage of 16 V which was heldfor 100 minutes.

Test Results:

Sintered density: 4.6 g/Cm³,

capacitance: 100 577 μFV/g

leakage current: 0.73 nA/μFV.

EXAMPLE 17

High purity optical grade Ta₂O₅ was calcined first at 1700° C. for 4hours and thereafter for 16 hours at 900° C. to provide for more compactand coarser precursor particles. Physical properties of the pentoxidepowder are:

Average particle size (FSSS): 20 μm

bulk density (Scott): 39 g/inch³

Screen analysis: 400-500 μm  8.7% 200-400 μm 63.6% 125-200 μm 15.0%80-125 μm  7.2% 45-80 μm  3.8% <45 μm 1.7%

 Morphology is shown in FIGS. 11A-11C (SEM-pictures).

The oxide powder was reduced to metal as described in example 16,however at 1000° C. for 6 hours.

Leaching and P-doping was as in example 16.

The Tantalum Powder Showed the Following Analytical Data:

Average particle size (FSSS): 2.8 μm,

bulk density (Scott): 28.9 g/inch³,

BET surface: 2.11 m²/g,

flowability through nonvibrated funnel with 60°-angle and 0,1 inchopening: 25 g in 35 seconds,

Master Sizer D10=103.29 μm, D50=294.63 μm, D90=508.5 μm,

morphology: see FIGS. 12A-12C (SEM-pictures).

Chemical Analysis:

O: 7350 ppm

N: 207 ppm

H: 174 ppm

C: 62 ppm

Mg: 9 ppm

Fe: 5 ppm

Cr: <2 ppm

Ni: <3 ppm.

P: 150 ppm

Capacitor anodes were prepared and anodized as in example 16.

Test Results:

Sintered density: 4.8 g/cm³

Capacitance: 89 201 μFV/g

Leakage current: 0.49 nA/μFV

A second series of capacitors were prepared the same way, however withsintering temperature raised to 1310° C.

Test Results:

Sintered density: 5.1 g/cm³

Capacitance: 84 201 μFV/g

Leakage current: 0.68 nA/μFV

EXAMPLE 18

Several samples, each approximately 25 grams, of WO₃, ZrO₂, and V₂O₃were reduced individually with gaseous magnesium at 950° C. for 6 hours.The reduction products were leached with dilute sulfuric acid to removeresidual magnesium oxide. The product was a black metal powder in eachcase. The tungsten and zirconium powders had oxygen contents of 5.9 and9.6 W/W % respectively, indicating that the metal oxides were reduced tothe metallic state.

The present process appears to represent the only demonstrated way ofmaking high quality chemically reduced niobium powder. The reduction ofthe metal oxide with a gaseous reacting agent, such as magnesium, asshown herein is thus particularly suitable for producing powders useableas metal-metal oxide capacitor substrates. Although the reductionprocess was carried out with the metal oxide in a bed in contact with asource of magnesium gas, the reduction can take place in a fluidizedbed, rotary kiln, flash reactor, multiple hearth or similar systemsprovided the magnesium or other reducing agent is in the gaseous state.The process will also work with other metal oxides or metal oxidemixtures for which the reduction reaction with gaseous magnesium orother reducing agent has a negative Gibbs free energy change.

There are several advantages to the gaseous reduction processesdescribed herein. Treatment of the reduction products is much lesscomplicated and expensive than post reduction work-up of tantalum powderproduced by liquid phase reactions such as the sodium reduction ofK₂TaF₇ in a molten salt system. No fluoride or chloride residues areproduced in the present process. This eliminates a potentially seriousdisposal problem or the need to institute an expensive waste recoverysystem. The reduction of metal oxides with gaseous reducing agents givespowders with much higher surface areas than powders produced by themolten salt/sodium reduction process. The new process easily makespowders with very high surface area compared to the traditional method;the potential for making very high performance capacitor grade powdersis great with magnesium or other gaseous reducing agent.

The present invention further for the first time demonstrates thesuperiority of Ta-Nb alloy powders for use in the production ofcapacitors.

FIG. 16 shows the ratio of maximum obtainable capacitance (μFV/g) andBET-surface of powder (m²/g) in relation to the alloy composition. A andC represent pure Ta-, Nb-powders, respectively, as measured in presentExample 16. B represents the highest known values of pure Ta powdercapacitors as disclosed in Examples 2, 5 and 7 of WO 98/37249. Line 1represents expectable values for alloy powder capacitors from linearinterpolation from pure Ta, and Nb powder capacitors. E represents afictive Nb-powder capacitor wherein the insulating oxide layer has thesame thickness per volt as in Ta powder capacitors, however, thedielectric constant of niobium oxide differs. Line 11 represents linearinterpolation between B and E. D represents a measured value of 25 wt.-%Ta/75 wt.-% Nb alloy powder capacitor as presented in present Example15. Curve III represents the estimated dependency of capacitance onalloy composition of alloy powder capacitors in accordance with thepresent invention.

FIG. 13 is a block diagram of steps for achieving an electrolyticcapacitor usage of the invention. The steps comprise reduction of metaloxide with gaseous reducing agent; separation of reduction agent oxidefrom a mass of resultant metal; breakdown to powder form and/or primarypowder particle size; classification; optionally, presinter to establishagglomerated secondary particles (controlled mechanical methods andcontrol of original reduction or separation steps also being affectiveto establish agglomerates); deoxidation to reduce the oxygenconcentration; compaction of primary or secondary particles to a porouscoherent mass by cold isostatic pressing with or without use ofcompacting binders or lubricants; sintering to a porous anode form(which can be an elongated cylindrical, or slab or of a short lengthfrom such as a chip); anode lead attachment by embedding in the anodebefore sintering or welding to the sintered anode compact; forming theexposed metal surfaces within the porous anode by electrolytic oxidationto establish a dielectric oxide layer; solid electrode impregnation byimpregnating precursors into the porous mass and pyrolysis in one ormore stages or other methods of impregnation; cathode completion; andpackaging. Various additional steps of cleaning and testing are notshown. The end product is illustrated (in a cylindrical form) in FIG. 15as a Ta or Nb (or Ta—Nb—alloy) capacitor 101 in partial cut-away form asa porous Ta or Nb (or Ta—Nb alloy) anode 102, impregnated with a solidelectrolyte, surrounded by a counter-electrode (cathode) 104 andpackaging sheath 105 with a dense lead wire 106 of Ta or Nb (generallymatching the powder composition) that is joined to the anode by a weldjoint 107. As stated above, other known per se capacitor forms(different shape factors, different metals, different electrolytesystems anode lead joinder, etc.) are accessible through the presentinvention.

FIG. 14 is a block diagram collectively illustrating production of someof the other derivative products and uses of the invention including useof the powders as slips, in molding and loose pack form for furtherreaction and/or consolidation by way of sintering, hot isostaticpressing (H.I.P.) or in sinter/H.I.P. methods. The powders per se and/oras consolidated can be used in making composites, in combustion, inchemical synthesis (as reactants) or in catalysis, in alloying (e.g.ferrometallurgy) and in coatings. The consolidated powders can be usedto make mill products and fabricated parts.

In some instances the end use products made using the gas reductionproduced powders will resemble state of the art powders made with stateof the art (e.g. reduced) powders and in other instances the productswill be novel and have unique physical, chemical or electricalcharacteristics resulting from the unique forms as described herein ofthe powders produced by reduction by gaseous reducing agents. Theprocesses of going from powder production to end product or end use arealso modified to the extent the powders, and methods of producing thesame, produce modified impurity profiles and morphology.

The mill products and fabricated parts manufacture can involveremelting, casting, annealing, dispersion strengthening and other wellknown per se artifacts. The end products made through further reactionof the metal powders can include high purity oxides, nitrides, silicidesand still further derivatives such as complex ceramics used inferroelectrics and in optical applications, e.g. perovskite structurePMW compounds.

It will now be apparent to those skilled in the art that otherembodiments, improvements, details, and uses can be made consistent withthe letter and spirit of the foregoing disclosure and within the scopeof this patent, which is limited only by the following claims, construedin accordance with the patent law, including the doctrine ofequivalents.

What is claimed is:
 1. Niobium powder in the form of agglomeratedprimary particles with a particle size of 100 to 1000 nm, wherein theagglomerates have a particle size corresponding to D10=3 to 80 μm,D50=20 to 250 μm and D90 30 to 400 μm as determined by Mastersizer. 2.Niobium powder according to claim 1, containing up to 40 at.-% of Taalone or with one or more of at least one metal selected from the groupof Ti, Mo, W, Hf, V and Zr, based on the total metal content.
 3. Niobiumpowder according to claim 2, containing at least 2 at.-% of the othermetal(s).
 4. Niobium powder according to claim 2, containing at least3.5 at.-% of the other metal(s).
 5. Niobium powder according to claim 2,containing at least 5 at.-% of the other metal(s).
 6. Niobium powderaccording to claim 2, containing at least 10 at.-% of the othermetal(s).
 7. Niobium powder according to claim 2, containing up to 34at.-% of the other metal(s).
 8. Niobium powder according to one of claim2, containing tantalum as the other metal.
 9. The powder according toclaim 1, wherein the powder is in the form of agglomerated substantiallyspherical primary particles having a diameter ranging from 100 to 1500nm.
 10. The powder according to claim 1, wherein the powder has aBET-surface value and an alloy density value and the multiplicationproduct of the BET-surface value and the alloy density value ranges from8 to 250 (m²/g)×(g/cm³).
 11. The powder according to claim 1, whereinthe powder has an agglomerate particle size ranging from 20 to 300μ asdetermined as D50-value according to Mastersizer.
 12. The powderaccording to claim 1, wherein the powder contains (i) oxygen in anamount ranging from 2500 to 4500 ppm/m², (ii) up to 10,000 ppm nitrogen,(iii) up to 150 ppm, carbon, and (iv) less than a total of 500 ppmimpurity metals.
 13. Niobium powder according to either of claim 1 or 2,which after sintering at 1100° C. and forming at 40 V exhibit a specificcapacitor capacitance of 80,000 to 250,000 μFV/g and a specific leakagecurrent density of less than 2 nA/μFV.
 14. An alloy powder for use inthe manufacture of electrolyte capacitors consisting essentially ofniobium and containing up to 40 at.-% of tantalum based on the totalcontent of Nb and Ta.
 15. The powder according to claim 14, containingat least 2 at.-% of tantalum.
 16. The powder according to claim 15,containing at least 3.5 at.-% of tantalum.
 17. The powder according toclaim 15, containing at least 5 at.-% of tantalum.
 18. The powderaccording to claim 15, containing at least 10 at.-% of tantalum.
 19. Thepowder according to claim 14, containing from 12 to 34 at.-% oftantalum.
 20. The powder according to claim 14, wherein the powder is inthe form of agglomerated substantially spherical primary particleshaving a diameter ranging from 100 to 1500 nm, wherein the primaryparticles have a BET surface value and a density value, and wherein themultiplication product of the BET surface value and the density valueranges from 15 to 60 (m²/g)×(g/cm³).
 21. The powder according to claim14, wherein the powder has a mean particle size D50-value according toMastersizer ranging from 20 to 250 μm.
 22. A capacitor anode obtained bysintering of a powder in accordance with claim 14, and anodization. 23.A capacitor comprising an anode according to claim
 22. 24. A process forthe manufacture of alloy powder according to claim 14, comprising thesteps of (a) Hydriding an electron-beam melted alloy ingot containing Nband up to 40 at.-% Ta based on the total content of Nb and Ta, and (b)Comminuting said hydrided alloy ingot, and (c) Dehydriding thecomminuted alloy obtained from step (b), and (d) Forming said comminutedalloy into flakes, and (e) Agglomerating said flakes at a temperature of800 to 1150° C. in the presence of an alkali earth metal as a reducingagent, and (f) Leaching and washing the agglomerated alloy flakes toremove any residual and residual product of the reducing agent.
 25. Theprocess according to claim 24, wherein during the agglomeration step thealloy powder is doped with phosphorous and/or nitrogen.
 26. A processfor making a metal powder selected from the group consisting of Ta, Nb,Ta alloys, Nb alloys, and combinations thereof, alone or with one ormore of metals selected from the group consisting of Ti, Mo, W, Hf and Vand Zr, wherein the process comprises: (a) providing an oxide or mixedoxides of the metal(s), wherein the oxide or the mixed oxides are in aform that is traversable by gas, (b) passing a hydrogen-containing gasthrough an oxide mass at an elevated temperature at a first stage andremoving at least 20% of the oxygen contained in the oxide mass, therebyreducing the oxide or mixed oxides to a suboxide, and (c) reducing thesuboxide in a second stage with a reducing agent selected from the groupof reducing metals and hydrides of reducing metals, and thereby freeingmetal from the suboxide and forming a primary metal powder.
 27. Theprocess of claim 26, wherein in step (c), the suboxide is substantiallyor completely reduced.
 28. The process according to claim 26, whereinthe reducing agent is selected from the group consisting of Mg, Ca, Al,Li, Ba, Sr, hydrides of Mg, Ca, Al, Li, Ba, Sr, and combinationsthereof.
 29. The process according to claim 26, wherein the primarymetal powder is processed to an agglomerated secondary form.
 30. Theprocess according to claim 29, wherein a deoxidation step is applied tothe agglomerated secondary form of the powder.
 31. The process accordingto claim 26, wherein the process further comprises subjecting theprimary metal powder to deoxidization by exposing the powder to agaseous reducing agent.
 32. The process according to claim 26, whereinthe first stage is carried out until the volume of solid matter isreduced by at least 35 to 50%.
 33. The process according to claim 26,wherein the reduction in the first stage is conducted with MeO_(x),wherein Me denotes Ta and/or Nb and x assumes a value of 1 to
 2. 34. Theprocess according to claim 26, wherein the reduction product from thefirst stage is maintained at approximately the reduction temperature fora further 60 to 360 minutes.
 35. The process according to claim 26,wherein Mg, Ca and/or hydrides thereof are used as reducing agents inthe second stage.
 36. The process according to claim 26, wherein themetal comprises tantalum and the oxide comprises tantalum pentoxide. 37.The process according to claim 26, wherein the metal comprises niobiumand the oxide comprises niobium pentoxide or a niobium suboxide.
 38. Theprocess according to claim 26, wherein the oxide contains tantalum in anamount of up to 50 atomic % based on the total content of metals. 39.The process according to claim 26, wherein the form of the oxide masstraversable by gas provides a void volume of at least 90%.
 40. Theprocess according to claim 26, wherein the oxide is provided in the formof agglomerated primary oxide particles with diameters from 100 to 1000nm and an average agglomerate size ranging from 10 to 1000 μm.
 41. Theprocess according to claim 26, wherein the reducing agent is magnesium.42. The process according to claim 26, wherein the elevated temperatureduring passing the gaseous reducing agent through the oxide mass isbelow 0.5, the melting point of metal powder.
 43. The process accordingto claim 26, wherein the temperature is below 0.4, the melting point ofthe primary metal powder.
 44. The process according to claim 26, whereinthe primary metal powder is subjected to a further deoxidation treatmentto produce a finished metal product.
 45. The process according to claim26, wherein one or more finishing deoxidation steps are provided as anextension of the reduction reaction.
 46. The process according to claim26, wherein the finishing deoxidation step is a separate treatment. 47.The process according to claim 26, wherein the metal powder is furtherformed into a coherent porous mass.
 48. A single-stage process formaking a metal powder selected from the group consisting of Ta and Nb,and one or more metals selected from the group consisting of Ti, Mo, W,Hf, V and Zr, the process comprising: (a) providing an oxide or mixedoxides of the metal(s), wherein the oxide or the mixed oxides are in aform that is traversable by gas, (b) generating a gaseous reducing agentat a site outside an oxide mass and passing the gas through the mass ata first temperature, and (c) reducing of the oxide(s) at a secondtemperature and freeing metal portion from the oxide, wherein the firsttemperature is the same or less than the second temperature.
 49. Theprocess of claim 48, wherein the oxide in step (c) is substantially orcompletely reduced.
 50. The process of claim 48, wherein the processavoids using a molten or a solid reducing agent.
 51. The process ofclaim 48, wherein residual oxide of reducing agent formed in thereaction is easily removed.
 52. The process of claim 48, wherein a highsurface area powder is formed in a process that essentially avoids useof molten state reducing agent in production of metal or alloy powder.53. A capacitor anode comprising a sintered niobium powder agglomeratedprimary particles with a particle size ranging from 100 to 1000 nm,wherein the agglomerated primary particles have a particle size rangingfrom D10=3 to 80 μm, D50=20 to 250 μm and D90 30 to 400 μm as determinedby Mastersizer.
 54. The capacitor anode of claim 53, wherein the anodeis made by a process comprising (i) sintering a powder in the form ofagglomerated primary particles with a particle size of 100 to 1000 nm,wherein the agglomerates have a particle size corresponding to D10=3 to80 μm, D50=20 to 250 μm and D90 30 to 400 μm as determined byMastersizer and (ii) subjecting the powder to anodization.
 55. Acapacitor comprising an anode according to claim
 53. 56. The capacitoraccording to claim 53, wherein the capacitor is a solid electrolytecapacitor.
 57. An alloy powder for use in the manufacture of electrolytecapacitors comprising niobium and containing up to 40 atomic % oftantalum based on the total content of Nb and Ta.
 58. The powderaccording to claim 57, wherein the powder contains at least 2 atomic %of tantalum.
 59. The powder according to claim 57, wherein the powdercontains at least 3.5 atomic % of tantalum.
 60. The powder according toclaim 57, wherein the powder contains at least 5 atomic % of tantalum.61. The powder according to claim 57, wherein the powder contains atleast 10 atomic % of tantalum.
 62. The powder according to claim 57,wherein the powder contains from 12 to 34 atomic % of tantalum.
 63. Thepowder according to claim 57, wherein the powder is in the form ofagglomerated substantially spherical primary particles having a diameterof 100 to 1500 nm, the powder has a BET surface value and a densityvalue and the multiplication product of the BET surface and the densityranges from 15 to 60 (m²/g)×(g/cm³).
 64. The powder according to claim57, wherein the powder has a mean particle size D50-value according toMastersizer of 20 to 250 μm.
 65. A process for the manufacture of alloypowder for use in the manufacture of electrolyte capacitors comprisingniobium and containing up to 40 atomic % of tantalum based on totalcontent of Nb and Ta comprising: (a) hydriding an electron-beam meltedalloy ingot containing Nb and up to 40 atomic % Ta based on the totalcontent of Nb and Ta, (b) comminuting said hydrided alloy ingot, (c)dehydriding the comminuted alloy obtained from step (b), and (d) formingsaid comminuted alloy into flakes, (e) agglomerating said flakes at atemperature of 880 to 1150° C. in the presence of an alkali earth metalas a reducing agent, and (f) leaching and washing the agglomerated alloyflakes to remove any residual and residual product of the reducingagent.
 66. The process according to claim 65, wherein during theagglomeration step the alloy powder is doped with phosphorous and/ornitrogen.